The sarcoplasmic reticulum (SR) of smooth muscle is endowed with two different types of Ca²⁺ release channels, i.e. inositol 1,4,5-trisphosphate receptors (IP₃Rs) and ryanodine receptors (RyRs). In general, both release channels mobilize Ca²⁺ from the same internal store in smooth muscle. While the importance of IP3Rs in agonist-induced contraction is well established, the role of RyRs in excitation-contraction coupling of smooth muscle is not clear. The participation of smooth muscle RyRs in the amplification of Ca²⁺ transients induced by either opening of Ca²⁺-permeable channels or IP₃-triggered Ca²⁺ release has been studied. The efficacy of both processes to activate RyRs by calcium-induced calcium release (CICR) is highly variable and not widely present in smooth muscle. Although RyRs in smooth muscle generate Ca²⁺ sparks that are similar to those observed in striated muscles, the contribution of these local Ca²+ events to depolarization-induced global rise in [Ca²⁺]_i is rather limited. Recent data suggest that RyRs are involved in regulating the luminal [Ca²⁺] of SR and also in smooth muscle relaxation. This review summarizes studies that were carried out mainly in muscle strips or in freshly isolated myocytes, and that were aimed to determine the physiological role of RyRs in smooth muscle.